The invention relates to a spark plug for internal combustion engines. A spark plug is already known (U.S. Pat. No. 4,540,910) in which an intermediate layer is arranged between the metal electrode, which consists of a nickel alloy, and a highly wear-resistant metal coating consisting of a platinum containing alloy, which intermediate layer serves to compensate for the sharply diverging heat expansion behavior of the metal electrode and the metal coating; this intermediate layer consists of an alloy which is composed of a platinum alloy and nickel. In order to apply the wear-resistant metal coating to the metal electrode, the wear-resistant metal coating is first mechanically plated together with the intermediate layer and the intermediate layer, provided with the metal coating is then connected with the metal electrode by resistance welding.
A spark plug in which a coating of noble or precious metal such as platinum is attached to the free end face of the center electrode by resistance welding has also already been described in the DE-PS No. 31 32 814. In this center electrode, however, there is the problem that the noble metal layer becomes detached from the center electrode because of stresses in the connecting region during higher thermal and corrosive loads.
The DE-PS No. 22 56 823 shows a spark plug with a center electrode whose end face is protected against high wear by a platinum piece. This platinum piece is provided with an intermediate layer plated thereon in a similar manner to the example according to the U.S. Pat. No. 4,540,910 cited above; this intermediate layer comprises a material which possesses the same or approximately the same high resistance to temperature and corrosion and thermal expansion characteristic as the center electrode itself. This intermediate layer consists of a nickel-base alloy of which the center electrode itself can also be composed. This intermediate layer is connected with the center electrode by welding, particularly by resistance welding. When using such spark plugs in internal combustion engines, it has also been found out that the fastening of the platinum pieces on the electrodes is not satisfactory.